The present invention relates to airfoils designed to delay the onset of the drag rise associated with transonic flight.
The parasite drag on an aircraft includes both viscous drag, resulting from the viscosity of the air, and pressure drag, resulting from an imbalance of pressures acting on the aircraft surfaces. At subsonic velocities, the viscous drag is the predominant contributor to the parasite drag of the aircraft. However, as the aircraft approaches Mach 1.0, local regions of supersonic flow develop on the surface of the aircraft. For local Mach numbers nearly equal to 1.0, the air is able to recompress (return to subsonic conditions) without forming local pressure jump discontinuities or shocks on the surface of the aircraft. At higher aircraft Mach members, local regions of supersonic flow develop for which it is no longer possible for the air to re-compress without a local pressure jump, or shock wave. This increased pressure drag is referred to as wave drag. Under these circumstances, the pressure drag becomes a sizsable portion of the total parasite drag of the aircraft, and increases dramatically with only a small increase in aircraft velocity. The dramatic increase in drag caused by the onset of wave drag is generally called the drag rise because the drag coefficient rises almost exponentially with only a small increase in velocity as the shock waves forming on the aircraft strengthen. Most commercial jet aircraft operate in the transonic region, sufficiently below Mach 1.0 to avoid the drag rise, because it is very inefficient to attempt to propel the aircraft at a velocity at which wave drag is a significant component of the total drag.
Operation of an aircraft at transonic speeds presents a particular problem to the designer of the airfoils which provide lift and control to the aircraft. Such airfoils provide lift by accelerating the air over the upper surface of the airfoil to reduce the relative pressure on the upper airfoil surface. Unfortunately, the acceleration of air over the upper surface of the airfoil on an aircraft already operating at transonic speeds requires that the flow of air over the airfoil become supersonic. It is important to delay the onset of wave drag on a transonic airfoil in spite of the fact that the flow of air over the airfoil must be at least partially supersonic to allow the aircraft to fly as close to Mach 1.0 as possible without encountering an unacceptable and uneconomic increase in drag.
An airfoil designed to be efficient at transonic speeds is illustrated in U.S. Pat. No. 3,952,971 to Whitcomb, entitled "Airfoil Shape for Flight at Subsonic Speeds". The Whitcomb airfoil is typical of transonic airfoils in that the forward 60% of the airfoil is nearly symmetric, with the upper and lower surfaces having a very flat profile. The upper surface maintains its flat profile over the entire extent of the airfoil, tapering downwardly at the aft end. The lower surface has a more complex shape over the aft 40% of the airfoil, curving upwardly and then downwardly to merge with the upper surface at the trailing end of the airfoil.
The relatively flat upper surface of the Whitcomb airfoil results in the formation of a shock wave which is sufficiently weak so that wave drag is minimized and the onset of drag rise is delayed to higher speeds. The curvature of the aft 40% of the lower surface of the airfoil causes the wing to have a high degree of camber in this region, which provides a significant portion of the total lift. The symmetric nature of the forward 60% of the airfoil limits the ability of the Whitcomb airfoil, and transonic airfoils in general, to generate lift. Basically, transonic airfoils such as Whitcomb are effective at delaying the drag rise to a velocity higher than the design speed of the aircraft, but at the expense of lift performance relative to other types of airfoils.
An attempt to improve the lift performance of a transonic airfoil is illustrated in U.S. Pat. No. 4,858,852 to Henne and Gregg. Henne and Gregg add lift to the airfoil by providing divergence between the upper and lower surfaces of the airfoil at the trailing end, with the airfoil terminating at a relatively blunt base. This configuration has the desired characteristic of improving lift and delaying the onset of the drag rise, but increases the base drag. In addition, providing divergence to the upper and lower surfaces at the aft end means that the thinnest portion of the airfoil is at some distance from the trailing end. The diverging surfaces at the trailing end result in high relative pressures on the lower surface of the airfoil aft of the section of minimum thickness. These high pressures generate high hinge moments about the section of minimum thickness, which is typically the weakest point of the airfoil. Preserving the structural integrity of the airfoil without adding excessive weight is a particular problem in the Henne and Gregg design because of the large moments applied to the airfoil at its weakest point.